1. Field of the Invention
The present invention relates to charge generation layers (CGLs) and charge transport layers (CTLs) which are prepared by coating a substrate with a coating solution prepared by dissolving a binder, a specified organosilane, and either a charge generation material (CGM) or a charge transport material (CTM) in a suitable solvent. The present invention also relates to organic photoconductive imaging receptors which contain such a CGL and/or CTL and processes for preparing such a CGL and CTL.
2. Discussion of the Background
A general discussion of electrophotography (photocopying) is given in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th ed, vol. 9, pp. 245-277, Wiley, N.Y. (1994), and a brief description of laser beam printing is provided in Encyclopedia of Electronics, 2nd ed, Gibilisco et al, Eds., pp. 669-671, TAB BOOKS, Blue Ridge Summit, Pa. (1990), both of which are incorporated herein by reference.
Photoreceptors are the central device in photocopiers and laser beam printers. In most photocopiers and laser beam printers, the photoreceptor surface is contained on the outside surface of a hollow metal cylinder, called a drum. Typically, the drum is made of a metal, such as aluminum, which may be anodized or coated with a thin dielectric layer (injection barrier) which is in turn over coated with photogeneration and photoconduction layers.
Key steps in transfer electrophotography include the charging step, the exposure step, the development step, and the transfer step. In the charging step, ions are deposited on the surface of the photoconductor drum. In the exposure step, light strikes the charged photoreceptor surface and the surface charges are neutralized by mobile carriers formed within the photoreceptor layer. Thus, the charge on the surface is transmitted in the exposed areas of the photoconductive layer to the oppositely charged metal substrate of the drum. In the development step, a thermoplastic pigmented powder (toner) which carries a charge is brought close to the photoreceptor so that toner particles are directed to the charged image regions on the photoreceptor. In the transfer step, a sheet of paper is brought into physical contact with the toned photoreceptor, and the toner is transferred to the paper by applying a charge to the backside of the paper.
Presently, the most suitable photoconductive imaging receptors for low and medium speed electrophotographic plain-paper copiers and laser printers have a double-layered configuration. Photogeneration of charge carriers (electron-hole pairs) takes place in a thin charge generation layer (CGL), typically 0.1 to 2.0 .mu.m thick, which is coated on a conductive substrate such as an aluminum alloyed drum. After photogeneration, mobile carriers (usually holes) are injected into a thicker charge transport layer (CTL), which is about 10 to 40 .mu.m thick and coated on top of the CGL, under an electric field gradient provided by a negative surface charge. These holes drift to the outermost layer of the photoreceptor to selectively neutralize surface charge, thereby forming a latent electrostatic image, which is subsequently developed by thermoplastic toner.
The physical durability of the organic photoconductive imaging receptor is the major characteristic that determines service lifetime, and such durability depends on the mechanical properties of the surface CTL. The CTL is formulated from two major components. They are electron-donor molecules responsible for hole transport, known as the charge-transport material (CTM), and an appropriate binder resin, which must be amorphous and transparent to light. The CTM is usually a low molecular weight organic compound with arylamine or hydrazone groups, and it is selected primarily on the basis of solubility, compatibility with the binder resin, charge transport property, and electrophotographic cyclic stability. The CTM is a non-reactive binder resin diluent (molecular dopant), and it must be compatible in approximately equal parts by weight with the binder resin to ensure good charge mobility, which involves electron hopping between adjacent molecules of the CTM.
The role of the binder resin is to impart the physical durability necessary for acceptable lifetime under the service conditions encountered in copiers and printers. It is well known that the most suitable binder resins belong to the general class of aromatic polycarbonates (PCR), which exhibit such desirable characteristics as solubility (to allow film coating from solution), high carrier mobility, compatibility with the CTM, transparency, durability, adhesion to the CGL, and so on. The simplest and best known example is bisphenol-A polycarbonate (BPA-PCR), more formally called poly[2,2-bis-(4-phenylene)propane carbonate], which has good impact strength and toughness.
Organic photoconductive imaging receptors are conveniently prepared by the dip-coating process, in which a substrate is dipped into a first solution which contains a solvent in addition to the ingredients of the CGL and, after drying, the CGL-coated substrate is dipped into a second solution which contains a solvent in addition to the ingredients of CTL. However, when forming a CTL by the dip-coating process, the thickness of the CTL is not uniform near the edge of the CTL at the point where the substrate was not immersed in the solution. Specifically, the thickness of the CTL increases from the edge until it reaches a plateau, which represents the thickness of the CTL over the bulk of the CTL. The distance between the edge of the CTL and the point where the thickness of the CTL levels off is referred to as the drop zone. To maximize the production of the CTL, it is desired to minimize the drop zone.
Moreover, it has been found that certain CTMs do not exhibit the same solubility in PCRs as others. This can lead to poor adhesion between the substrate and the CGL/CTL.
Thus, there remains a need for improved CGLs and CTLs which exhibit improved adhesion to the substrate. There also remains a need for organic photoconductive imaging receptors which contain such a CGL and/or CTL and processes for preparing such a CGL and/or CTL and such organic photoconductive imaging receptors.